1. Field of the Invention
The present invention is directed to a nickel-based solder for high-temperature soldered joints of steels and nickel-based alloys.
2. Description of the Prior Art
Nickel based solders are primarily employed for high-temperature soldered joints of different types of steels and nickel-based alloys. Because of the unavoidable embrittling intermetallic compounds, nickel solders which are currently known are only manufactured in powder form, or as a solder paste produced from the powder, or as amorphous ribbons.
German Patent 27 55 435 discloses a uniform, ductile solder foil for hard-soldering of metal parts which is manufactured using a known rapid-quenching technique. The solder consists of 0 through 4 atomic percent iron, 0 through 21 atomic percent chromium, 0 through 16 atomic percent boron, 0 through 19 atomic percent silicon, 0 through 22 atomic percent phosphorus, and the remainder nickel, with possible contaminants. The total content of iron, chromium and nickel should lie in the range from 76 through 84 atomic percent, and the total content of boron, silicon and phosphorus should lie in the range from 16 through 24 atomic percent. The foil is essentially vitreous, i.e., amorphous.
Another ductile hard solder foil is described in European Published Application 0 056 141 having at least 50 percent vitreous structure, and consisting of palladium, boron and nickel. This solder consists of 1 through 41 atomic percent palladium, 0 through 20 atomic percent chromium, 11 through 20 atomic percent boron, and the remainder nickel. The total content of nickel, palladium and chromium is in the range of 80 through 89 atomic percent.
These and similar alloy compositions are essentially suitable for hard soldering at high temperatures of austenitic, martensitic and ferritic stainless steels, as well as nickel-based alloys. Soldering at high temperatures, i.e., above 1000.degree. C., is required for component parts which are exposed to such high temperatures during use. Examples are components of aircraft turbines, and metallic exhaust catalyzers.
When soldering thin materials, the material around the soldering joint is significantly changed due to the diffusion of the solder into the material. As a result, properties of the material are usually drastically degraded. Included among these properties, in addition to the mechanical properties, are embrittlement and the corrosion properties such as, for example, a deterioration of the oxidation properties. This problem is especially complicated in high-temperature materials as are employed, for example, in exhaust catalyzers for motor vehicles.
Fe-Cr-Al alloys have proven particularly well-suited as a carrier material for such exhaust catalyzers because of their high resistance to oxidation. In order to maintain the flow-through resistance in the catalyzer at a low level, thin bans approximately 50 .mu.m in thickness are used, from which the carrier member is wound, forming a honeycomb member. For stability, soldering of the individual winding layers to each other is required. This has proven extremely difficult, and such soldered joints have proved unsatisfactory, for the following reasons. Soldering using powder solder requires a considerable technological outlay, because wetting and contamination by the solder at locations which are not to be soldered is unavoidable. The use of a precise dose of the solder in powder form is not possible, and as a result a significant change in the alloy composition of the carrier material, which is only about 50 .mu.m thick, can occur at the soldered location. Solder pastes are manufactured from powder solders with an organic binder, which may constitute up to 50 percent of the solder paste. The use of such pastes is thus limited by this binder, since it must burn during soldering. The wetting capability of the solder is thereby degraded, and a contamination of the soldered location with carbon cannot be avoided. Soldering with amorphous solder bands is unsatisfactory because the extremely high yielding point of the amorphous solder ribbon can considerably complicate further processing. The use of an amorphous solder ribbon presents considerable difficulties whenever a cutting or breaking of the component is required in further processing. A high boron or phosphorus content, as is necessary for vitrification in amorphous alloys, is undesirable in a solder alloy. This is in part because the relatively large percentage of those elements in the alloy necessarily requires that other elements be present in lower percentages, which thereby deteriorates the mechanical properties and the oxidation properties of the alloy to a considerable degree. Lastly, due to the high nickel content in the solder, austenite forms in the carrier material and in the environment of the soldered joint. This austenite deteriorates the oxidation properties of the soldered joint, so that a premature oxidation through the joint can be seen.
Previous attempts have been made to solve these problems by using optimally small quantities of solder. Due to the diffusion during soldering, however, the nickel in the solder is distributed substantially equally over the solder material and the carrier material in the region of the soldered joint. In the aforementioned soldering of the catalyzer carrier, wherein the carrier band has a thickness of only 50 .mu.m, only a small amount of material is available for such diffusion processes. In order to obtain a sufficient nickel concentration in the catalyzer carrier, so that no conversion to form austenite occurs, it is usually sufficient merely to maintain the quantity of solder low. This could be achieved by using especially thin amorphous solder foils.
It has therefore been attempted to use an amorphous solder foil having a band thickness of only 15 .mu.m. This attempted solution, however, has serious disadvantages. The vitrifying elements such as boron which are present in such amorphous alloys drastically deteriorate the oxidation properties. As noted above, the high yielding point of the amorphous solder ribbons is also unfavorable. Even when such a thin foil as described above is used, problems still arise if a cutting or breaking of the solder foil is required.